It is known that in current Diesel engines with direct injection systems the temperature of the intake air is increased by compression, so that during the compression run of the piston the injection system injects fuel at the point in which combustion is desired.
This is relatively simple, yet there are certain drawbacks such as emission of pollutants, as these depend greatly on the temperature of said combustion. In general terms, one can say that combustion at low temperatures produces soot particles, smoke, etc., while at high temperatures it produces NOx.
As regards injection systems, manufacturers are working with injectors allowing nulti-injections at increasingly high pressures, which enables an outstanding control of the amount of fuel and the injection point.
Within this frame it can be seen that all improvements made in these injection systems are limited in practice by the air management systems, as however precise the control of the injected fuel is, unless there is an equally precise control of the air mass introduced in the cylinder and its temperature when the intake valve is closed it will not be possible to optimise combustion nor, therefore, polluting emissions cycle by cycle.
Air management is even more difficult when exhaust gas re-circulation systems (EGR) participate, taking hot air from the exhaust and sending it to the intake.
However, heating intake air with resistors is used frequently, and in this sense can be cited patents WO00/34643; U.S. Pat. No. 6,152,117 and U.S. Pat. No. 5,988,146, which describe heating resistors, although they are meant for use in Diesel engines of large size where sturdiness is more important than performance, performance being understood here as short pre-heating times, as well as low load losses and a self regulation capacity with a response time on the order of characteristic engine times (the inverse of the engine revolution rate), etc. On the other hand, these resistors provide no information on the temperature at which they operate, which makes it impossible to perform a closed loop regulation so that it is not possible to operate at high temperatures (500-1,200° C.), since at these temperatures a control failure can result in the destruction of the resistor.
To implement the regulation function with the heaters described in the aforementioned patents, a standard thermocouple can be welded to them; however, in addition to the cost and industrialization problems associated to this, this entails functionality problems as if the thermocouple welded is made of small section wires it will not be very sturdy and the weld will break easily with the dilations and contractions of the resistor as it is successively heated and cooled, as well as due to the vibrations of the engine; it should be kept in mind that the element is placed at the inlet of the intake pipes, on the cylinder head and very near the cylinders.
If the above-described problem is to be solved, one must weld a thermocouple made of wires with a relatively large section. However, the heat will then flow from the reading point through the thermocouple cable, so that the reading will be false. Moreover, welding a thermocouple to the resistor and placing a piece of wire in the path of the airflow creates a singularity in the surface of the resistor, at the measurement point, which makes the temperature measured erroneous in any event.
In view of the above, it would be desirable to be able to set the temperature of the intake air and know the airflow at each inlet of the intake pipes.